Method for selective deposition of metal thin film

ABSTRACT

A metal thin film is deposited on predetermined portions of an underlayer of a substrate by a chemical deposition method with good selectivity, good reproducibility and high deposition rate by preventing hydrogen atoms from the adhesion to portions of the substrate not to be deposited with a metal using a special means for heating only the substrate or a special gas flow controlling means.

BACKGROUND OF THE INVENTION

This invention relates to a process and an apparatus for selectivedeposition of a metal thin film such as a tungsten thin film on aspecified area of a substrate, particularly with high selectivity andhigh rate.

With higher integration of LSI, there have become finer individualelements and wiring, or diameters of contact-holes or through-holesformed on insulating films for contacting wiring with each other. On theother hand, since the thickness of insulating films cannot be reduced, aratio of the depth to the diameters of these holes (aspect ratio)becomes larger, which results in making it remarkably difficult to fillup the holes with a conductive metal. For example, according to asputtering method of aluminum which is the most general method forforming a metal film, the diameter of 2 μm and the aspect ratio of about1 is the upper limit of filling up. In order to fill up holes havingsmaller diameter and larger aspect ratio, other methods should beapplied. One of these methods is a selective chemical vapor deposition(CVD) method of a metal, typically tungsten (W), which method has beenstudied and many reports have been published.

According to a selective CVD method of W, tungsten hexafluoride (WF₆)and hydrogen (H₂) are used as starting material gases, which areintroduced with a predetermined pressure and in a certain amount into areactor wherein a heated substrate is installed. On the portionsexposing silicon (Si) underlayer of the substrate, a W film is depositedby the following Si reduction reaction:

    WF.sub.6 +3/2Si→W+3/2SiF.sub.4 ↑              (1).

By the Si reduction reaction, the W film can be grown up to severalhundreds angstroms in thickness. Further, aluminum (Al) can also form aW film by directly reacting with WF₆ like Si. Since a catalytic actionas to the adsorption and dissociation of H₂ takes place on the conductorfilm such as W film formed by the formula (1), the following reductionreactions proceed by H atom to continuously grow the film:

    H.sub.2 →2H (on catalyst surface)                   (2)

    WF.sub.6 +6H→W+6HF                                  (3).

Further, since adsorption and dissociation of H₂ of the above formula(2) take place even on a conductor film of MoSi₂, WSi₂, PtSi, etc., a Wfilm is deposited and grows. The above-mentioned reaction proceeds at asubstrate temperature of about 200° C. or higher.

On the other hand, the Si reduction reaction of (1) does not take placeon an insulating film of SiO₂, Si₃ N₄, Al₂ O₃ or the like. Further,since the catalytic action as to the adsorption and dissociation of H₂at about 700° C. or less on such an insulating film does not take place,the dissociation of H₂ by the formula (2) does not take place to proceedthe reduction reaction by the H atom, so that no metal thin film isformed. Therefore, according to the selective CVD method using a metalhalide such as WF₆, or the like and H₂ as starting material gases, ametal thin film is selectively deposited on a underlayer of Si or aconductor metal, so that it is possible in principle to fill up holeshowever fine and deep these holes may be so long as the startingmaterial gases are supplied into the holes.

But, according to a prior art selective CVD method for a metal thinlayer, there takes place undesirably a phenomenon that a metal isdeposited even on an insulating film of SiO₂, or the like.

As an apparatus for the selective CVD method for forming a metal thinfilm typified by a W thin film, there has been used a low pressure CVDapparatus excellent generally in a film thickness distribution and stepcoverage properties. In such a case, considering the selective formationof a metal thin film, it was necessary to make some device so as not toform a metal thin film on a reactor wall or the like other than thesubstrate. As the low pressure CVD apparatus, there are a hot-wall typeand a cold-wall type.

The hot-wall type CVD apparatus is characterized by heating the wholereactor with a heater, and has an advantage in that infrared light fromthe heater transmits the reactor, the interior of which is heateduniformly. Further, in the case of forming a metal thin layer by using ametal halide gas such as WF₆, or the like and H₂, the selective filmformation becomes possible by using quartz which suppress the formationof the metal thin layer as the reactor. But there is a problem in thatwhen there are contaminations which become nuclei for film formation onthe inner wall of the reactor even in trace amounts, the film formationarea of metal thin film is enlarged around the nuclei as their centersand the metal thin film is finally formed on portions of the substratenot desired to be formed.

On the other hand, the cold-wall type CVD apparatus is characterized bycooling the whole reactor with water, while heating a substrate with aninfrared lamp from a back side of the substrate, on the desired portionof front side of which is formed a metal thin film, together withsubstrate supporting units. According to a process for using such acold-wall type CVD apparatus, there are advantages in that since heatedportions other than the desired substrate surface on which a metal thinfilm is to be formed are not exposed to the starting material gases, thereaction between the reactor wall and the starting material gases doesnot take place and the film-forming rate is stable. Further, since thesubstrate is heated together with the substrate supporting units, thereis an advantage in that the substrate surface temperature can bemaintained uniformly. But there is a problem in that since the substratesupporting units are also heated, a metal thin film is also formed onthe surface of substrate supporting units and a metal thin filmformation area is enlarged therefrom so as to form a metal thin film onundesired portions of the substrate.

As mentioned above, according to the prior art processes, it wasdifficult to form thin films selectively on only the desired portionswhile maintaining good selectivity with good reproducibility withoutfail. In order to improve the selectivity in the prior art processes, itis possible to employ as general consideration a low temperature for thetreatment (lower than 350° C.), a short deposition time, carefulcleaning of a substrate surface, a small surface for deposition, etc.But there may bring about lowering in throughput and a limitation to theapplications for processes. This is contrary to the desire to carry outselective film formation for obtaining any desired film thickness with ahigh film-forming rate while maintaining good selectivity. Selective CVDof W is disclosed, for example, in J. Electrochemical Society, vol. 131(1984), pp. 1427-1433; Proc. 2nd. Int. IEEE VLSI MultilevelInterconnection Conf. vol. 25 (1985), pp. 343, etc. Further, apparatusfor selective CVD of W is disclosed, for example, in U.S. Pat. No.4,547,404, etc.

Further, the formation of a metal thin film on an insulating film ofSiO₂ or the like using a metal halide gas and H₂ as the startingmaterial gases is difficult as mentioned above in principle. But asdisclosed in Extended Abstracts of the Meeting of 170th Electrochem.Soc. vol. 86-2, pp. 500 (1986, Oct.), when H atoms are produced in a gasphase by using H₂ plasma, etc., a metal thin film can easily be formedeven on an insulating film of SiO₂, or the like. That is, since there isno catalytic action of adsorption and dissociation of H₂ on theinsulating film of SiO₂ or the like, the dissociation reaction of theabove-mentioned formula (2) does not take place. But by adhering H atomsproduced in the gas phase to the insulating film of SiO₂ or the like,the reaction of formula (3) proceeds even on the surface of SiO₂ or thelike to form a metal thin film of W. In other words, when H atoms arepresent in the gas phase from some causes and adhere to a surface ofSiO₂ portions on which the formation of W or the like film is notdesired, a metal is deposited thereon. But according to prior artselective CVD for forming metal thin films and apparatus used therefor,the prevention of the adhering of H atoms to surface portions on whichthe formation of the film is not desirable was not considered.

As mentioned above, according to the prior art technique, the preventionof degradation of selectivity, that is, the prevention of adhering of Hatoms to an insulating film, this being a cause for depositing the metalon SiO₂ or the like insulating film on which the deposition of the metalis not desirable, was not considered and the metal thin film was notformed with good reproducibility, good selectivity and high rate.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a process and an apparatusfor forming a metal thin film rapidly with good reproducibility whilemaintaining good selectivity and overcoming the defects of prior arttechnique mentioned above.

This invention provides a process for selectively depositing a metalthin film on surfaces of predetermined portions of a substrate by achemical vapor deposition method in a reactor, which comprises using asstarting material gases a metal halide gas and a hydrogen gas, andheating at least the portions to be deposited with a metal thin layer toa temperature sufficient for reacting with the starting material gases,whereby the reaction with the starting material gases is not broughtabout on the portions of the substrate surface where the deposition ofmetal is not desired by preventing the adhesion (or adsorption) ofhydrogen atoms thereto, and the reaction with the starting materialgases is brought about for film formation on the portions of thesubstrate surface where the deposition of metal is desired.

This invention also provides an apparatus for selectively depositing ametal thin film on surfaces of predetermined portions of a substrate bya chemical vapor deposition method, characterized in that there isprovided in a reactor a means for introducing starting material gasescomprising a metal halide gas and a hydrogen gas, and a means forheating at least the portions to be deposited with a metal thin layer toa temperature sufficient for reacting with the starting material gases,whereby the reaction with the starting material gases is not broughtabout on the portions of the substrate surface where the deposition ofmetal is not desired by a means for preventing the adhesion of hydrogenatoms thereto, and the reaction with the starting material gases isbrought about for film formation on the portions of the substratesurface where the deposition of metal is desired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a whole apparatus for explaining oneexample of this invention.

FIG. 2 is an enlarged and partly broken perspective view of surroundingportions of a substrate (wafer) of FIG. 1.

FIG. 3 is a schematic view of a whole apparatus for explaining anotherexample of this invention.

FIG. 4 is an enlarged and partly broken perspective view of surroundingportions of a substrate (wafer) of FIG. 3.

FIGS. 5 and 6 are schematic views of whole apparatus used for conductingexperiments comparing with the Examples of this invention.

FIG. 7 is a cross-sectional view of a surface portion of a substrateshowing behaviors of H atoms at the time of selective filling up of ametal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to this invention, hydrogen atoms are prevented from adheringto portions of a substrate surface where the formation of a metal thinfilm is not desired, that is, an insulating film of SiO₂ or the like,said adhesion of H atoms being a cause for degrading the selectivity.When the surface of insulating film such as SiO₂ or the like film has atemperature showing no catalytic action for adsorption and dissociationof H₂, no metal is deposited so long as H atoms are not adhered (oradsorbed) to the SiO₂ or the like insulating film.

Routes of H atoms for adhering to an insulating film in a prior artmetal thin film selective CVD are shown in FIG. 7. A process forpreventing H atoms from adhering to an insulating film is explainedbelow referring to FIG. 7 in the most typical example of W selectiveCVD. FIG. 7 is a cross-sectional view of a substrate surface, whereinnumeral 16 denotes SiO₂, numeral 17 denotes Si, numeral 18 denotes W,and numerals 101, 102 and 103 denote H atoms adhering to SiO₂ by variousroutes respectively. According to the first route (101), H atomsproduced by adsorption and dissociation of H₂ at places other than thesubstrate, that is, mainly a heated reactor inner wall in the case ofthe hot-wall type CVD apparatus, and mainly heated substrate supportingunits in the case of the cold-wall type CVD apparatus, are desorbed fromthe surface thereof, and diffuse and adhere to SiO₂ portions. Accordingto the second route (102), H atoms produced by adsorption anddissociation at the surfaces of W which is formed in contact holes aredesorbed from the surfaces thereof, and diffuse and adhere to SiO₂portions. According to the third route (103), H atoms produced byadsorption and dissociation at the surfaces of W as mentioned abovereach SiO₂ portions by surface diffusion (spillover H atoms). Amongthese routes, a phenomenon caused by the H atoms adhered to the SiO₂portions by the third route reflects on the overflowing shape of W fromcontact holes which are filled up with W, but is quite different frominsular W particles which are observed as a phenomenon of degradation inselectivity under ordinary film-forming conditions of W selective CVD.Such a phenomenon can be solved by regulating the pressure of startingmaterial gases in the reactor and suppressing the surface diffusion of Hatoms, so that there arises no problem. Therefore, in order to maintaingood selectivity and to form W thin film rapidly with goodreproducibility, it is necessary to suppress the first route H atomscompletely by the methods mentioned below. Further, in order to suppressthe second route H atoms, it is necessary to conduct the film formationat a temperature lower than the temperature at which the H atoms beginto desorb from the surfaces of W having filled the holes. The desorptionof H atoms from the W surfaces of contact holes becomes more difficult,when the temperature becomes lower. But from the viewpoint of highdeposition rate, to lower the temperature unnecessarily low is notpreferable. Since the kinetic energy of H atoms present on the Wsurfaces takes the Boltzman distribution, the number of H atoms desorbedfrom the W surfaces does not change rapidly with a threshold value at acertain temperature and thus cannot be reduced to zero even if thetemperature is lowered as low as possible.

The film formation with complete maintainance of selectivity means thateven if W film is formed in contact holes as desired (usally 70 to 100%of the hole depth), W particles are not deposited on SiO₂ portionspractically. Therefore, the upper limit of the heating temperaturevaries with changes of the opening area of contact hole portions and theopening rate of the hole portions against SiO₂ portions.

The number of H atoms subjecting to desorption and diffusion from the Wsurfaces at contact hole portions per unit time is compared at atemperature T₁ (K) and a temperature T₂ (K) as shown below. A ratio ofthe number of H atoms desorbed from a unit area per unit time at T₁(N_(H) (T₁)) to the number of H atoms desorbed from a unit area per unittime at T₂ (N_(H) (T₂)) is represented by the following equation:##EQU1## wherein n_(H) (T₁) is the number of H atoms present on thesurface generated by adsorption and dissociation per unit area at thetemperature T₁, n_(H) (T₂) is the number of H atoms present on thesurface generated by adsorption and dissociation per unit area at thetemperature T₂, ΔE is an activation energy for desorption (Kcal/mol),and R is a gas constant (Kcal/mol. K).

On the other hand, when the adsorption and dissociation of H₂ moleculeson W are a rate controlling step of film formation reaction, a ratio offilm formation rate at a temperature T₁ to that at a temperature T₂(i.e. R(T₁)/R(T₂)) is represented by the following equation: ##EQU2##

Since the time necessary for film formation to the predeterminedthickness is inversely proportional to the film formation rate, a ratioof the number of H atoms desorbed from a unit area at the temperature T₁until the end of film formation (N'_(H) (T₁)) to the number of H atomsdesorbed from a unit area at the temperature T₂ until the end of filmformation (N'_(H) (T₂)) is represented by the following equation takingthe equations (4) and (5) into consideration: ##EQU3##

Here, the activation energy for desorption (ΔE) is equal to the bondenergy Do (W-H) of the adsorbed H atom and W (W-H) on the W metalsurface. It is known from literature that ΔE=78 (Kcal/mol). When T₁=550° C. (=823 K), T₂ =600° C. (=873 K), the ratio of H atom numbersdesorbed from the W surface until the end of film formation becomes1:17. This value changes depending on the difference in the bond energyDo (Me-H) (Me: a metal atom). For example, when surfaces of bottoms(that is, an underlayer) of contact holes is made of Pt, ΔE=66(Kcal/mol) which value is smaller than W, so that the ratio of H atomnumbers desorbed from the Pt surface becomes 1:10. This means that thedesorption reducing effect by temperature lowering is small comparedwith the case of W.

In an example mentioned below, mention is only made on a limitedreaction system under limited reaction conditions. But the prevention ofadhering of H atoms to the SiO₂ portions by the second route can only beattained fundamentally by lowering the substrate temperature. In anexample of selective W film formation mentioned below, the adhesion of Hatoms to the SiO₂ portions by the second route is prevented completelyby making the substrate temperature 590° C. or lower, preferably, 580°C. or lower, more preferably 550° C. to 250° C.

According to this invention, so long as the adhesion of H atoms by thesecond route is prevented, that is, film formation is carried out at atemperature lower than the temperature at which H atoms begin to desorbfrom the surface of W formed in the holes, there can be attained theobject of forming a metal thin film rapidly with good reproducibilitywhile maintaining the good selectivity mentioned above, provided thatthe adhesion of H atoms by the first route is completely prevented.Therefore, the prevention of adhesion of H atoms by the first routebecomes an important problem. This is explained below in detail.

There are two methods for preventing the adhesion of H atoms by thefirst route, wherein H atoms, which are produced by adsorption anddissociation of hydrogen molecules at the inner wall of the reactor, thesubstrate supporting units, and the like other than the substrate,desorb from these surfaces, and diffuse and adhere to the SiO₂ portionsas shown in FIG. 7.

A first method of the two is to heat only the substrate and to remove acause for generating H atoms by suppressing the temperatures ofsurfaces, which contact with the starting material gases, such as theinner wall of the reactor, the substrate supporting units, and the likeother than the substrate as low as possible, that is, the temperaturelower than the desorption temperature of H atoms, for example 200° C. orlower, more preferably 100° C. to 0° C. A second method of the two is toprevent H atoms, which are desorbed and diffused from the surfaces otherthan the substrate, from reaching portions of the substrate on which thedeposition of a metal is not desired by controlling the gas flows.

According to the first method, it is preferable to use a light sourceemitting light which can be absorbed by the substrate material and themetal formed as a means for heating the substrate. Portions other thanthe substrate is prevented from irradiation of light from the lightsource, and even if irradiated, the temperature rise is prevented by acooling means such as water cooling. Thus, the generation of H atoms atportions other than the substrate is prevented.

According to the second method, the heating means is not a problem. Itis important to form a cooled gas flow controlling means for separatinga space which contacts with the surface of heated substrate on which ametal thin film is to be formed selectively, from a space which contactswith the surfaces of inner wall of the reactor, the substrate suportingunits, and the like, from which H atoms can be generated, by retaining afine gap along the periphery of the substrate. Into an inner spacesurrounded by the substrate and the gas flow controlling plate, thestarting material gases including no H atoms dissociated are introduced.These gases are evacuated by passing through the fine gap along theperiphery of the substrate to give a high flow rate, which prevents theH atoms generated in spaces other than the above-mentioned space fromreaching the surface portions of the substrate on which the depositionof metal is not desired. Since the reaction pressure in a selective CVDfor metal thin film by using an ordinary low-pressure CVD apparatus is0.1 Torr or more, the gas flow is a viscous flow. Therefore, the flowrate of the starting material gases passing through the fine gapmentioned above after the reaction is at least 3 times, usually severaltimes as much as the diffusion rate of H atoms, the reaching of H atomsby diffusion to the insulating film such as SiO₂, etc. on the substrateon which the formation of a metal thin film is not desired is prevented.The flow rate v (cm/sec) passing through the fine gap can be representedby the following equation: ##EQU4## wherein V (sccm) is the flow amountof starting material gases; P (Torr) is the pressure; r (cm) is thediameter of the substrate; and d (cm) is the distance of the gap.Provided that V=500 (sccm), P=10 (Torr), r=5.0 (cm), and d=0.1 (cm), theflow rate v=1006 (cm/sec) is obtained.

On the other hand, when the diffusion coefficient of H atoms is D_(p)(cm² /sec) at a pressure P, and the time is t (sec), the diffusiondistance L (cm) is represented by the equation: ##EQU5## As is clearfrom the equation (8), since the diffusion distance is not proportionalto the time in the first degree, it is impossible to define thediffusion rate per unit time. Provided that t=1 (sec), L≈2 to 7 (cm) isobtained, and thus the diffusion rate of 2 to 7 (cm/sec) is obtainedfrom the distance of 2 to 7 cm per unit time of 1 sec. Under theabove-mentioned conditions, since the flow rate passing through the finegap mentioned above after the reaction of the starting material gases isseveral hundreds times as fast as the diffusion rate of H atoms, theadhesion of H atoms to the SiO₂ portions can sufficiently be prevented.

This invention is further illustrated referring to the attacheddrawings. A W thin film is selectively formed on only a lot of contactholes (0.6 to 2.0 μm□, depth 1.1 μm, opening rate 1/16 to 1/4) formed ina silicon (Si) substrate covered with a thermal oxide film (SiO₂) usingtungsten hexafluoride (WF₆) and hydrogen (H₂) as the starting materialgases. An example is carried out by using an apparatus shown in FIG. 1,wherein the substrate is only heated and the desorption and adhesion ofhydrogen atoms is prevented by cooling portions of inner wall of thereactor, subtrate supporting units, gas inlet pipes and other innersurfaces in the reactor exposed to the starting material gases otherthan the substrate. On the other hand, another example is carried out byusing an apparatus shown in FIG. 3, wherein the adhesion of H atoms isprevented by controlling gas flows so as to prevent hydrogen atomsgenerated by adsorption and dissociation of hydrogen molecules atportions in the reactor other than the substrate from reaching theportions of the substrate not to be deposited with a metal.

In FIG. 1, numeral 1 denotes a WF₆ bomb, numeral 2 denotes a H₂ bomb,numerals 3 and 4 denote mass flow controllers, respectively, numerals 5and 6 denote stop valves for the gases, respectively, numeral 7 is ahalogen lamp attaching a reflective mirror cooled with water, and awafer (substrate) 12 is heated by the light irradiated from the lamp andpassed through an irradiation window 9 made of quartz. Numeral 8 denotesa reactor cooled with water so as to suppress the temperature rise ofthe inner wall by irradiation of light. Further, the reactor isconnected to a vacuum evacuation system (not shown in the drawing) inthe direction A. In order to suppress the temperature rise of theirradiation window 9 and inner wall of the reactor 8 when the halogenlamp 7 is lighted, water-cooled shading plates 10 and 11 are providedoutside and inside of the reactor 8, respectively, and the irradiationwindow 9 is cooled by air blown from the direction B. Further, the wafer12 is supported by water-cooled substrate supporting means 13 so as tomake almost point contact at 3 points so as not to lower the substratetemperature.

The shading plate 11 and the substrate supporting means 13 arewater-cooled to a temperature of 200° C. or lower, preferably 100° C. to0° C. In addition, the shading plate 11 provided in the reactor 8 ismade so as to diffuse the starting material gases effectively on thewafer by connecting to the inlet 29 of the starting material gases. InFIG. 1, numeral 14 is a manometer and numeral 15 is a power controller.

FIG. 2 is an enlarged perspective view showing the periphery portions ofthe wafer 12 of FIG. 1 in detail. In FIG. 2, the shading plate 11 andthe wafer 12 are partly broken so as to make the understanding easy. InFIG. 2, numeral 27 denotes a pipe for water cooling the shading plate,numeral 28 denotes a pipe for water cooling the substrate supportingmeans.

In this Example, since the vapor-phase growth (or deposition) is carriedout by using an experimental apparatus, the temperature of wafer 12 ismeasured by a thermocouple directly attaching a 0.25 φ thermocouple tothe wafer 12 using a ceramic adhesive. Further, the temperature of wafer12 is controlled by monitoring the voltage output C from thethermocouple and changing the output of the halogen lamp 7 by the powercontroller 15. The pressure in the reactor 8 is controlled by monitoringit by a capacitance manometer 14 and changing the conductance of theevacuation system (not shown in FIG. 1) in the direction A.

Next, the formation of W for filling up only contact holes formed in asilicon wafer is explained in accordance with the procedures.

The reactor 8 is evacuated so as to make a vacuum of 10⁻³ Torr or less.After the evacuation, H₂ gas is introduced into the reactor 8 and ahalogen lamp is lighted simultaneously to begin the heating of thesubstrate (silicon wafer). The flow amount of H₂ is controlled by themass flow controller 4 to 500 sccm. The pressure in the reactor 8 ismaintained at 10 Torr by the conductance controller of the evacuationsystem (not shown in FIG. 1) and the temperature of the substrate 12 isset at 550° C. by the power controller 15 of the halogen lamp 7. Whenboth the pressure and temperature are stabilized, WF₆ is introduced intothe reactor. The flow amount of WF₆ is maintained at 3 sccm by the massflow controller 3. By the introduction of WF₆, slight changes in thepressure and the temperature are observed, but the predetermined valuesare recovered again after several seconds by the controllers,respectively. After 2.5 minutes from the introduction of WF₆, the supplyof each gas is stopped by the stop valves 5 and 6. At the same time, thehalogen lamp is put out to stop the heating of the substrate 12 and theresidual gases in the reactor are evacuated. After cooling the substrateto 100° C. or lower, the reactor is leaked to an atmospheric pressureand the substrate 12 is taken out of the reactor 8. By the aboveprocedure, the contact holes with 1 μm deep are filled up with W withabout 0.9 μm in film thickness. The selectivity is evaluated by cuttingthe wafer 12 and observing the peripheries of contact holes with ascanning electron microscope (SEM). According to the above-mentionedExample, the contact hole portions were filled up with W almostcompletely, while SiO₂ portions 16 around the holes were not changed atall. This means that the selectivity is very good.

The pressure and the temperature mentioned in the above Example are onlyone example, respectively. There can be used a pressure of 0.2 to 10Torr and a temperature of 590° C. or lower, more preferably 250° to 550°C. The flow rate of H₂ may be changed from 100 to 500 sccm and that ofWF₆ from 3 to 50 sccm, while maintaining the ratio of the flow rates (H₂/WF₆) from 20/1 to 200/1. By employing the above-mentioned conditions,the metal (W) thin film can be deposited with good selectivity. There isa tendency to lower the film formation rate when the temperature and thepressure are lowered as in a prior art process, but according to thisinvention, the upper limits of the temperature and the pressure can beraised remarkably as mentioned above.

In the next place, the selectivity is evaluated by repeating theabove-mentioned Example except for changing the substrate temperature to600° C. and film formation time to 2 minutes. The observation by SEMrevealed that on SiO₂ 16 portions around contact holes, insular tungstenparticles were formed to degrade the selectivity.

This degradation in the selectivity seems to be caused by H atoms whichare generated by adsorption and dissociation on the surface of thetungsten 18 in the contact hole portions of the substrate surface,adhered to the SiO₂ portions 16 by desorption and diffusion. The numberof H atoms desorbed from the hole portion W surface 18 which degrade theselectivity by adhering to the SiO₂ portions and are counted from theinitial time of film formation till the completion of film formation isabout 17 times at 600° C. statistically as large as those at 550° C.from the equation (6). Since the degradation in selectivity does notchange rapidly around a threshold value, it is difficult to determine atemperature at which the selectivity is clearly degraded. But, from theabove-mentioned Examples, it is most preferable to conduct the filmformation at about 550° C. or lower in order to maintain the practicallyusable selectivity. Even if conducted at 550° C., the film-forming rateis 360 nm/min from the above-mentioned Examples, said value beingimproved by several times to several tens times compared with prior artprocesses and practically usable.

Another Example is explained referring to FIG. 3 wherein even if H atomsare generated at portions other than the substrate when heated, theadhesion of H atoms to the substrate is prevented by controlling the gasflows.

In FIG. 3, explanation for the same numerals as used in FIG. 1 isomitted for simplicity. Numeral 19 denotes a bomb of inert gas such asAr, He, N₂, or the like which does not pertain to the reaction, numeral20 denotes a flow regulator, numeral 21 denotes a stop value, numeral 22denotes a water-cooled gas flow controlling means such as a plate andnumeral 23 denotes a substrate stage including a heater for heating thesubstrate. WF₆ and H₂ introduced from the bombs 1 and 2, respectively,are flowed out of only a small gap (usually 0.1 to 10 mm, preferably 1to 8 mm, in this Example about 1 mm) between the gas flow controllingplate 22 and the substrate stage 23 and evacuated by the evacuationsystem (not shown in FIG. 3) in the direction of A. Therefore, H atomsgenerated at the surface of the substrate stage 23 outside of thesubstrate do not diffuse and adhere to the surface of substrate 12 viathe gap between the gas flow controlling plate 22 and the substratestage 23. FIG. 4 is an enlarged perspective view showing the peripheryportions of the wafer 12 of FIG. 3. In FIG. 4, the gas flow controllingplate 22 is partly broken so as to make the understanding easy. In FIG.4, numeral 30 is a pipe for water-cooling the gas flow controllingplate, numeral 31 denotes a gas introducing pipe, and numeral 32 denotesa pipe for a manometer. The temperature of the substrate 12 iscontrolled by uniformly heating the whole substrate by the thermalconductivity obtained from the heat of the heated substrate stage 23carried by the inert gas introduced from the bomb 19 and passed to thegap between the substrate stage 23 surface and the rear side ofsubstrate through the gas inlet 24 for thermal conductivity. Since thegas flow controlling plate 22 is water cooled at a temperature of 200°C. or lower, preferably 100° C. to 0° C., and its temperature rise dueto radiation heat from the substrate stage 23 is suppressed, no H atomsare generated.

The substrate temperature is measured by directly contacting thethermocouple with the substrate 12 in the same manner as mentioned aboveand the temperature control is conducted by changing a heater output ofthe heater included in the substrate stage 23. The pressure of theinside surrounded by the gas flow controlling plate 22, the substrateand the substrate stage 23 is controlled by monitoring with thecapacitance manometer 14 and changing the gap between the gas flowcontrolling plate 22 and the substrate stage 23 so as to change theconductance.

The formation of W for filling up only contact holes formed in a siliconwafer is explained in accordance with the procedures.

The reactor 8 is evacuated so as to make a vacuum of 10⁻³ Torr or less.At the same time, an electric current is passed through the heaterincluded in the substrate stage 23 to heat the substrate stage 23. Afterthe evacuation, 500 sccm of H₂ gas is introduced into the reactor 8 and200 sccm of Ar was flowed to the reactor 8 from the bomb 19 as a gas forthermal conductivity. the gap between the gas flow controlling plate 22and the substrate stage 23 is regulated so as to make the capacitancemanometer 14 show 10 Torr when 3 sccm of WF₆ and 500 sccm of H₂ areintroduced while maintaining the substrate stage 23 temperature at roomtemperature, so that the pressure sensor 14 shows about 9.5 Torr beforethe introduction of WF₆. When the temperature of the substrate 12 israised to 550° C. and stabilized, WF₆ is introduced into the reactor 8.After the introduction of WF₆, the reading of the manometer 14 increasesgradually but is stabilized at about 10 Torr. After 3 minutes from theintroduction of WF₆, the supply of each gas is stopped by individualstop values 5, 6, and 21. At the same time, the heater of the substratestage is turned off to stop the heating of substrate and the residualgases in the reactor are evacuated. After the substrate 12 is cooled to100° C. or lower, the reactor 8 is leaked and the substrate 12 is takenout. The selectivity is evaluated in the same manner as mentioned aboveby the SEM observation to admit that the selectivity is very good. Inthis Example, the same conditions as mentioned in the Example using theapparatus of FIG. 1 can also be employed.

The lower the pressure becomes, the larger the ratio of the startingmaterial gases passing through the fine gap along the periphery portionof the substrate after the reaction to the diffusion rate of H atomsbecomes, so that the employment of a lower pressure is advantageous formaintaining good selectivity.

The shape of the gas flow controlling means is not limited to that shownin FIGS. 3 and 4. Any shapes which exhibit the above-mentioned effectssuch as a cylinder, a semi-sphere, and the like, can be employed.

The next Example is carried out in order to ascertain whether theselectivity is degraded by the H atoms or not by using an apparatusshown in FIG. 5.

The apparatus of FIG. 5 is almost the same as that of FIG. 3 except thatthe starting material gases WF₆ and H₂ are introduced into the reactor 8separately and a H₂ introducing pipe is surrounded by a quartz tube 25so as to produce H₂ plasma by microwaves from the outside. Numeral 26denotes a microwave oscillator and numeral 27 denotes a microwaveresonator. When the film formation is carried out in the same manner asmentioned above using the apparatus of FIG. 3 while the power source forthe microwave oscillator is turned off, the same results are admitted,that is, the selectivity is very good.

In the next place, the power source of the microwave oscillator 26 isturned on and H₂ plasma produced is also introduced into the reactor 8as H atoms together with H₂ used in the Example using the apparatus ofFIG. 3. Since the plasma disappears above 3 Torr, the pressure ofplasmas is made 3 Torr. Further, since the plasma disappears below theoutput of lower than 20 W of the microwave oscillator, the plasma isproduced at the output of 20 to 80 W. In individual cases, W is formedon the SiO₂ portions 16 around the contact holes, so that theselectivity is degraded remarkably to be lost. With an increase of theoutput of the microwave oscillator 26, the surface of W film on the SiO₂16 portions becomes smooth from the rough state. This means that W isformed more uuniformly. Further, this phenomenon takes place on thewhole surface of the substrate.

The above-mentioned results show that when H atoms are formed in the gasphase, W is formed irrespective of the material of the surface ofsubstrate 12, and the more the H atoms present in the gas phase become,a uniform film of W can be formed even on the SiO₂ 16 surface, on whichthe formation of W is inherently difficult. This means that in order toform the W film selectively, it is necessary to remove the H atomspresent in the gas phase as completely as possible. On the other hand,since the lower end of the plasma discharge region is apart from thesubstrate 12 by 10 cm or more, it is not necessary to consider theinfluence of charged particles present in the plasma. Further, since thegas introduced is only H₂, neutral particles present other than H₂ areonly H atoms.

The next Example is carried out for studying why the selectivity isdegraded in the prior art W selective CVD method.

In this Example, the apparatus shown in FIG. 6 is used. The apparatus ofFIG. 6 is almost the same as that of FIG. 3 except that the gas flowcontrolling plate 22 is apart from the substrate 12 by 3 cm or more soas to make the presence of it negligible and the pressure in the reactor8 is controlled by changing the conductance of the evacuation system(not shown in FIG. 6) in the direction A. The procedures for the filmformation are carried out in the same manner as described in those usingthe apparatus of FIG. 3 except that the position for changing theconductance controlling the pressure in the reactor 8 is different andthe pressure at the film formation is 12.3 Torr due to the influence ofthe inert gas used for heating the substrate on the reading of thecapacitance manometer 14. This is because when the substrate stage 23 isat room temperature, and 3 sccm of WF₆ and 500 sccm of H₂ are introducedinto the reactor 8 and the conductance of the evacuation system isregulated so as to make the pressure 10 Torr, followed by theintroduction of 200 sccm of Ar, the reading of the capacitance manometer14 shows 12.3 Torr. Therefore, when the film formation is carried outunder a total pressure of 12.3 Torr, it may be considered that the filmformation is carried out under a pressure of about 10 Torr which is atotal of partial pressures of WF₆ and H₂, although the value may bechanged depending on the difference in viscosities of the gases. Whenthe film formation is carried out by using the apparatus shown in FIG. 6in the same manner as the process using the apparatus of FIG. 3, theselectivity is poor. Further, a lot of insular formation of W isobserved particularly on the SiO₂ portions 16 at the periphery portionof the substrate 12 and the number of insular W particles is reducedaccordingly near the central portion of the substrate. In the Exampleusing the apparatus of FIG. 5, the selectivity is degraded on the wholesurface of the substrate 12, while in the Example using the apparatus ofFIG. 6, the selectivity is degraded from the periphery portion of thesubstrate 12. This can be explained as follows. In FIG. 5, H atoms aregenerated in the gas phase considerally apart from the substrate 12 byabout 10 cm or more, and diffused and adsorbed on the whole surface ofthe substrate to degrade the selectivity. In contrast, in FIG. 6, Hatoms are generated and desorbed on the surface of the substrate stage23 having a metal surface with high temperatures, and diffused from theperiphery portion of the substrate 12 and adsorbed to degrade theselectivity.

From the results obtained by using the apparatus shown in FIGS. 3 to 6,it is revealed that the degradation in selectivity in the prior artprocesses is caused by the adsorption of H atoms on the SiO₂ portions16. By suppressing the generation of H atoms by using the apparatusshown in FIGS. 1 to 4, a metal thin film can be formed on a specialunderlayer of a substrate with a high rate of film formation (upto 360nm/min) while maintaining high selectivity.

This invention can be applied to any reaction systems for forming metalthin films by hydrogen reduction. As the metal halide gases, there canbe used WF₆, WCl₆, WCl₅, MoF₆, MoCl₅, TiCl₄, TaCl₅, NbCl₅, PtF₆, IrF₆,ReF₆, etc.

As the materials for surfaces of predetermined portions of the substrate(the underlayer), there can be used a metal such as Al, Cu, Ni, Cr, Mo,Pt, Ti, Si, or the like, a metal silicide such as WSi_(x), MoSi_(x),TiSi_(x), PtSi_(x), or the like, a metal nitride such as TiN_(y),WN_(y), or the like (in which x and y are production ratio of thecompounds), etc.

As the material for the portions other than the substrate, there can beused silicon oxide, silicon nitride, alumina, diamond or an organicinsulating film such as polyimide films (e.g. PIQ), etc.

By using these materials, corresponding metal thin films can be producedwith high selectivity.

As mentioned above, since metal thin films can be formed on specialsurface portions of substrates with good selectivity according to thisinvention, highly integrated LSI using W, Mo, or the like for filling upcontact holes and through-holes can be produced with improved yields andimproved reliability.

What is claimed is:
 1. A process for selectively depositing a metal thinfilm on surfaces of predetermined portions of a substrate by a chemicalvapor deposition method in a reactor, which comprises using as startingmaterial gases a metal halide gas and a hydrogen gas, and heating atleast the portions to be deposited with a metal thin layer to atemperature sufficient for reacting with the starting material gases,whereby the reaction with the starting material gases, is not broughtabout on the portions of the substrate surface where the deposition ofmetal is not desired by preventing the adhesion of hydrogen atomsthereto, the prevention of adhesion of hydrogen atoms being accomplishedby cooling portions other than the substrate of an apparatus in whichthe chemical vapor deposition takes place, and which are exposed to thestarting material gases, and the reaction with the starting materialgases is brought about for film formation on the portions of thesubstrate surface where the deposition of metal is desired.
 2. A processaccording to claim 1, wherein the heating to a temperature sufficientfor reacting with the starting material gases is carried out by using alight source emitting light which can be absorbed by the substratematerial and the metal formed.
 3. A process according to claim 1,wherein the metal halide is at least one member selected from the groupconsisting of WF₆, WCl₅, WCl₆, MoF₆, MoCl₅, TiCl₄, TaCl₅, NbCl₅, PtF₆,IrF₆, and ReF₆.
 4. A process according to claim 1, wherein the surfacesof predetermined portions of a substrate is made of a material whichreacts with the metal halide gas at a predetermined temperature to forma metal, or has a catalytic action for adsorption and dissociation ofhydrogen gas, and portions other than the substrate in the reactor aremade of a material having no catalytic action and not reacting with themetal halide gas at the above-mentioned predetermined temperature.
 5. Aprocess according to claim 4, wherein the material for the surface ofpredetermined portions of the substrate is silicon, a metal, a metalsilicide or a metal nitride, and the material for the portions otherthan the substrate is silicon oxide, silicon nitride, alumina, diamondor an organic insulating film.
 6. A process for selectively depositing ametal thin film on surfaces of predetermined portions of a substrate bya chemical vapor deposition method in a reactor, which comprises usingas starting material gases a metal halide gas and a hydrogen gas, andheating at least the portions to be deposited with a metal thin layer toa temperature sufficient for reacting with the starting material gases,whereby the reaction with the starting material gases is not broughtabout on the portions of the substrate surface where the deposition ofmetal is not desired by preventing the adhesion of hydrogen atomsthereto, and the reaction with the starting material gases is broughtabout for film formation on the portions of the substrate surface wherethe deposition of metal is desired wherein the prevention of adhesion ofhydrogen atoms is accomplished by controlling gas flows so as to preventhydrogen atoms generated by adsorption and dissociation of hydrogenmolecules at portions in the reactor other than the substrate fromreaching the portions of the substrate not to be deposited with a metal.7. A process according to claim 6, wherein the prevention of hydrogenatoms from reaching the portions of the substrate not to be depositedwith a metal is attained by(1) providing a cooled gas flow controllingmeans which separates a space contacting with surfaces of inner wall ofa reactor and substrate supporting units capable of generating hydrogenatoms, from a space contacting with the surface of substrate to bedeposited with a metal thin film with a fine gap along the peripheryportion of the substrate, (2) introducing the starting material gasescontaining no dissociated hydrogen atoms into an inner space surroundedby the substrate and the gas flow controlling means, and (3) making theflow rate of the starting material gases passing through the fine gapalong the periphery portion of the substrate after the reaction with thesubstrate surface at least 3 times as fast as a diffusion rate ofhydrogen atoms generated in portions other than the substrate so as toprevent the hydrogen atoms from reaching the portions of the substratenot to be deposited with a metal.
 8. A process according to claim 6,wherein the metal halide is at least one member selected from the groupconsisting of WF₆, WCl₅, WCl₆, MoF₆, MoCl₅, TiCl₄, TaCl₅, NbCl₅, PtF₆,IrF₆, and ReF₆.
 9. A process according to claim 6, wherein the surfacesof predetermined portions of a substrate is made of a material whichreacts with the metal halide gas at a predetermined temperature to forma metal, or has a catalytic action for adsorption and dissociation ofhydrogen gas, and portions other than the substrate in the reactor aremade of a material having no catalytic action and not reacting with themetal halide gas at the above-mentioned predetermined temperature.
 10. Aprocess according to claim 9, wherein the material for the surfaces ofpredetermined portions of the substrate is silicon, a metal, a metalsilicide or a metal nitride, and the material for the portions otherthan the substrate is silicon oxide, silicon nitride, alumina, diamondor an organic insulating film.